Gliomas
Glial cells, the most abundant cell type in the central nervous system, are cells that surround neurons and provide support for and insulation between them. Unlike neurons, glial cells do not conduct electrical impulses. There are two major classes of glial cells in the central nervous system: astrocytes and oligodendrocytes (Kandel E R, et al., Principles of Neural Science, 4th Ed. McGraw-Hill New York (2000), Ch. 2, pp. 20-21).
Glial cells in the vertebrate nervous system are divided into two major classes: microglia and macroglia. Microglia are phagocytes that are mobilized after infection, injury or disease, which arise from macrophages outside the nervous system. Three types of macroglial cells predominate in the vertebrate nervous system: oligodendrocytes, Schwann cells, and astrocytes. Astrocytes, the most numerous of glial cells in the central nervous system characterized by their star-like shape and the broad end-feet on their processes, are thought to play a nutritive role, and help form an impermeable lining in the brains capillaries and venules—the blood brain barrier—that prevents toxic substances in the blood from entering the brain. Oligodendrocytes, small cells with relatively few processes, and Schwann cells produce the myelin used to insulate nerve cell axons.
The term “glioma” encompasses all tumors thought to originate in the glial cell linage. (Veliz, I. et al., “Advances and challenges in the molecular biology and treatment of glioblastoma—is there any hope for the future?” Ann. Trans. Med. 3(1): 7. Doi: 10.3978/j.issn.2305-5939.2014.10.06. The location of the tumor depends on the type of cells from which it originates.
Malignant gliomas exhibit properties that resemble astrocytes or oligodendrocytes, hence the designation as astrocytomas and oligodendrogliomas. These tumors are graded on a scale from I to IV, based on how normal or abnormal the cells look. Of numerous grading systems in use, the most common is the World Health Organization (WHO) grading system for glioma (Louis D N, et al., Acta Neuropathol, 2007, 114(2):97-109). Grade I tumors are slow-growing, nonmalignant, and associated with long-term survival. Grade II tumors are relatively slow-growing but sometimes recur as higher grade tumors. They can be nonmalignant or malignant. Grade III tumors are malignant and often recur as higher grade tumors. Grade IV tumors reproduce rapidly and are very aggressive malignant tumors.
Low grade astrocytomas usually are localized and grow slowly. High grade astrocytomas grow at a rapid pace and are infiltrative. Astrocytomas can appear in various parts of the brain and nervous system, including the cerebellum, the cerebrum, the central areas of the brain, the brainstem, and the spinal cord.
Pilocytic Astrocytoma (also called Juvenile Pilocytic Astrocytoma), are grade I astrocytomas, which typically stay in the area where they started and do not spread. They are considered the “most benign” (noncancerous) of all the astrocytomas. Two other, less well known grade I astrocytomas are cerebellar astrocytoma and desmoplastic infantile astrocytoma.
Diffuse Astrocytoma (also called Low-Grade or Astrocytoma Grade II) (e.g., Fibrillary, Gemistocytic, Protoplasmic Astrocytoma) tend to invade surrounding tissue and grow at a relatively slow pace.
An anaplastic astrocytoma is a grade III tumor. These rare tumors require more aggressive treatment than benign pilocytic astrocytoma.
Astrocytoma Grade IV (also called Glioblastoma, previously named “Glioblastoma Multiforme,” “Grade IV Glioblastoma,” and “GBM”). There are two types of astrocytoma grade IV—primary, or de novo, and secondary. Primary tumors are very aggressive and the most common form of astrocytoma grade IV. The secondary tumors are those which originate as a lower-grade tumor and evolve into a grade IV tumor.
Subependymal Giant Cell Astrocytoma—Subependymal giant cell astrocytomas are ventricular tumors associated with tuberous sclerosis.
Oligodendrogliomas can be found anywhere within the cerebral hemisphere of the brain, although the frontal and temporal lobes are the most common locations. Sometimes oligodendrogliomas are mixed with other cell types. These tumors may be graded using an “A to D” system, which is based on microscopic features of the individual tumor cells. The grade indicates how quickly the tumor cells reproduce and how aggressive the tumor is. About 4% of primary brain tumors are oligodendrogliomas, representing about 10-15% of the gliomas. Only 6% of these tumors are found in infants and children. Most oligodendrogliomas occur in adults ages 50-60, and are found in men more often than women.
Mixed glioma (or oligoastrocytoma) usually contain a high proportion of more than one type of cell, most often astrocytes and oligodendrocytes. Occasionally, ependymal cells are also found. The behavior of a mixed glioma appears to depend on the grade of the tumor. It is less clear whether their behavior is based on that of the most abundant cell type.
Ependymal cells line the ventricles of the brain and the center of the spinal cord. These tumors are divided into four major types: subependymomas (grade I), typically slow growing tumors; myxopapillary ependymomas (grade I), typically slow growing tumors; Ependymomas (grade II), the most common of the ependymal tumors, which can be further divided into the following subtypes, including cellular ependymomas, papillary ependymomas, clear cell ependymomas, and tancytic ependymomas; and anaplastic ependymomas (grade III), typically faster growing tumors. The various types of ependymomas appear in different locations within the brain and spinal column. Subependymomas usually appear near a ventricle. Myxopapillary ependymomas tend to occur in the lower part of the spinal column. Ependymomas are usually located along, within, or next to the ventricular system. Anaplastic ependymomas are most commonly found in the brain in adults and in the lower back part of the skull (posterior fossa) in children. They are rarely found in the spinal cord. Ependymomas are relatively rare tumors in adults, accounting for 2-3% of primary brain tumors. However, they are the sixth most common brain tumor in children. About 30% of pediatric ependymomas are diagnosed in children younger than 3 years of age.
Optic gliomas may involve any part of the optic pathway, and they have the potential to spread along these pathways. Most of these tumors occur in children under the age of 10. Grade I pilocytic astrocytoma and grade II fibrillary astrocytoma are the most common tumors affecting these structures. Higher-grade tumors may also arise in this location. Twenty percent of children with neurofibromatosis (NF-1) will develop an optic glioma. These gliomas are typically grade I, pilocytic astrocytomas. Children with optic glioma are usually screened for NF-1 for this reason. Adults with NF-1 typically do not develop optic gliomas.
Gliomatosis Cerebri is an uncommon brain tumor that is most frequently pediatric and features widespread glial tumor cells in the brain. This tumor is different from other gliomas because it is scattered and widespread, typically involving two or more lobes of the brain. It could be considered a “widespread low-grade glioma” because it does not have the malignant features seen in high-grade tumors.
Glioblastoma
Glioblastoma multiforme (GBM), a WHO grade IV malignant glioma, classification name “glioblastoma”, is the most common and most aggressive primary brain tumor in adults. GBM accounts for 40%-60% of all diffuse astrocytic tumors and 10%-15% of all intracranial neoplastic lesions. The biological characteristics of this tumor are exemplified by prominent proliferation, active invasiveness, and rich angiogenesis. (Nakada, M. et al., Cancers, 2011, 3: 3242-3278). GBM is composed of poorly differentiated neoplastic astrocytes. The presence of microvascular proliferation and/or necrosis is essential for histopathological diagnosis of GBM.
GBM is one of the most aggressive human cancers and is very difficult to treat due to several complicating factors: the tumor cells are very resistant to conventional therapies; the brain is susceptible to damage by conventional therapy; the brain has a very limited capacity to repair itself; and many drugs cannot cross the blood-brain barrier to act on the tumor.
Although treatment can involve radiation, surgery and chemotherapy with temozolomide, which is an alkylating agent (P. J. Noughton et al. Clin. Cancer Res. (2000) 6:4110-4118), decades of surgical therapy, radiotherapy, and chemotherapy have failed to drastically change survival for GBM. The medium survival of patients with GBM in clinical trial populations treated with multimodal treatment approaches is approximately 12-15 months, with only 3%-5% of patients surviving longer than 36 months. (McNamara. M. G. et al., Cancers, 2013, 5: 1103-1119).
Glioblastoma multiforme has at least four distinct molecular subtypes. Tumor variants can be classified on the basis of somatic mutations in isocitrate dehydrogenase (IDH) ½ and TP53; transcriptional signature (classical, mesenchymal, neural or proneural), copy number variation, including co-deletion of chromosomes 1p and 19q; and amplification or mutation of the epidermal growth factor receptor (EGFR) and increased DNA hypermethylation of promoter-associated CpG islands. (Parker, N. R. et al., “Molecular heterogeneity in glioblastoma: potential clinical implications,” Frontiers in Oncology 5, article 55 (March 2015)).
Classical GBM tumors are characterized by abnormally high levels of epidermal growth factor receptor (EGFR) which is a protein found on the surface of some cells that, when bound by epidermal growth factor, sends signals for the cell to keep growing in number (proliferation). The Cancer Genome Atlas (TCGA) Research Network, Nature 455: 1061-1068 (2008).
EGFR abnormalities occur at a much lower rate in the three other GBM subtypes. The TP53 gene codes for tumor protein p53 that normally suppresses tumor growth. TP53 is rarely mutated in classical GBM tumors subtype, but is the most frequently mutated gene in other subtypes of GBM.
Proneural GBM tumors are characterized by alterations of platelet derived growing factor receptor A (PDGFRA) and point mutations in IDH1, the gene that encodes isocitrate dehydrogenase 1. The gene IDH1, when mutated, codes for a protein that can contribute to abnormal cell growth. PDGFRA, which plays an important role in cell proliferation, cell migration, and angiogenesis, was also found to be mutated and expressed in abnormally high amounts. PDGFRA alteration only occurs in Proneural tumors and not in any other subtypes. When PDGFRA is altered, too much of its protein can be produced, leading to uncontrolled tumor growth. The patients of this subtype tend to be younger and to survive longer than in other subtypes.
The Mesenchymal subgroup contains the most frequent number of mutations in the neurofibromatosis type 1 (NF1) tumor suppressor gene. Frequent mutations in the PTEN (phosphatase and tensin homolog) and TP53 tumor suppressor genes also occur in the Mesenchymal subgroup. PTEN protein acts as a tumor suppressor, helping regulate the cycle of cell division.
While the Neural subgroup has mutations in many of the same genes as the other groups, the group does not stand out from the others as having significantly higher or lower rates of mutations. The Neural group is characterized by the expression of several markers that are also typical of the brain's normal, noncancerous nerve cells, or neurons, such as NEFL, GABRA1, SYT1 and SLC12A5.
While classical and mesenchymal GBMs express gene expression profiles reminiscent of NSCs, IDH-mutant gliomas display a proneural phenotype. (Ilkanizadeh, et al., “Glial Progenitors as Targets for Transformation in Glioma,” Adv. Cancer Res. 121: 1-65 (2014)).
Based on clinical experience, two subgroups of otherwise histologically indistinguishable GBMs have been established: primary glioblastoma and secondary glioblastoma. Primary glioblastoma, which comprises more than 90% of biopsied or resected cases, arise de novo without antecedent history of low-grade disease, whereas secondary glioblastoma progresses from previously diagnosed low-grade gliomas. Primary glioblastomas display classical mesenchymal and neural phenotypes, whereas secondary glioblastomas tend to display a proneural phenotype that shifts toward a mesenchymal phenotype with recurrence. (Parker, N. R. et al., “Molecular heterogeneity in glioblastoma: potential clinical implications,” Frontiers in Oncology 5, article 55 (March 2015)).
These molecular subtypes of glioblastoma multliforme appear to differ in their clinical courses and therapeutic responses. For example, the different subtypes show varying responses to aggressive chemotherapy and radiotherapy, with a difference of around 50% between the subtypes. It has been suggested that the pathology of each subtype might begin from different types of cells, which might explain the variation in response to therapy. The greatest benefit was seen in the Classical and Mesenchymal subtypes, where intensive therapy has significantly reduced mortality; and there was a suggestion of efficacy in the Neural subtype; but the Proneural subtype was less responsive to intensive therapy including conventional chemotherapy or chemo-radiation therapy. (Verhaak, R G, et al., Cancer Cell, 17(1):98-110, 2010))
Major Glioma Signaling Pathways
Several major signaling pathways have been associated with Glioma (Nakada, M. et al., Cancers, 2011, 3: 3242-3278).
1. Receptor Tyrosine Kinase Pathway (RTK/PI3K/Akt/mTOR Pathway).
The RTK/P13K/Akt pathway regulates various fundamental cellular processes such as proliferation, growth, apoptosis, and cytoskeletal rearrangement. The pathway involves receptor tyrosine kinases (RTKs), for example, epidermal growth factor receptor (EGFR), platelet derived growing factor receptor (PDGFR), and vascular endothelial growth factor receptor (VEGFR), etc., as well as tumor suppressor protein phosphatase, for example, phosphatase and tension homolog (PTEN), and protein kinases PI3K, Akt, and mTOR. The Receptor Tyrosine Kinase pathway (RTK/PI3K/Akt/mTOR Pathway) is shown in FIG. 1.
EGFR gene amplification is the most frequent alteration (approximately 40%) in GBM. EGFR is a transmembrane glycoprotein member of the ErbB receptor family. In GBM, EGFR is dysregulated through overexpression, which arises because of EGFR gene amplification or activating mutations such as EGFRvIII that lead to ligand-independent signaling. EGFR aberrations have been correlated with a classical subtype of GBM (TCGA Research Network, Nature 455: 1061-68; Verhaak, Roel G. W. et al., Cancer Cell, 2010, 17: 98-110). Although it has been suggested that alterations of EGFR may be correlated with increased aggressiveness of GBM (Nakada, M. et al., Cancers, 2011, 3: 3242-3278), EGFR inhibitors (e.g., Gefinitib, Erlotinib) have not elicited clinical responses in patients with GBMs in clinical trials (Rich, J. N., et al., N. Engl. J. Med. 2004, 351, 1260-1261; Haas-Kogan, D. A. et al., J. Natl. Cancer Inst. 2005, 97, 880-887; van den Bent, M. J. et al., J. Clin. Oncol. 2009, 27, 1268-1274).
Overexpression of platelet-derived growth factor receptor (PDGFR), especially PDGFR-α and platelet-derived growth factor (PDGF) have been observed in astrocytic tumors of all grades, and their association with malignant progression has been suggested (Nakada, M. et al., Cancers, 2011, 3: 3242-3278). PDGFRA amplification (14%), as well as IDH1 mutation, are major features of the Proneural subtype of GBM according to the TCGA classification (TCGA Research Network, Nature 455: 1061-68; Verhaak, R. G. et al., Cancer Cell, 2010, 17: 98-110). Despite deep association of this molecule with GBM, anti-PDGFR therapy using Imatinib yielded only limited clinical responses (Reardon, D. A., et al., J. Clin. Oncol. 2005, 23, 9359-9368; Reardon, D. A., et al., Br. J. Cancer 2009, 101, 1995-2004).
The PI3Ks are widely expressed lipid kinases that promote diverse biological functions. The binding of PI3Ks and RTKs results in activation of Akt through phosphatidylinositol 3,4,5-triphosphate (PiP3) and 3-phosphoinositide dependent protein kinase-1 (PDK1), which affects multiple fundamental cellular processes including cell survival, proliferation, and motility. According to the integrated genomic classification of GBM, PI3K mutations (15%) are associated with the Proneural subtype (TCGA Research Network, Nature, 2008, 455: 1061-68; Verhaak, R. G. et al., Cancer Cell, 2010, 17: 98-110).
Decreased PTEN activity can activate the RTKs/PI3K/Akt pathway since PTEN negatively regulates the pathway by antagonizing PI3K function. Homozygous deletion or mutation of PTEN is a common genetic feature in GBM (40%), resulting in constitutive activation of the RTKs/PI3K/Akt pathway. PTEN loss is associated with both classical and mesenchymal subtypes of GBM, according to the TCGA study (TCGA Research Network, Nature 455(23): 1061-68).
Akt is an STK (Serine/threonine specific protein kinase) that regulates cell growth, proliferation, and apoptosis. Akt activation has been reported in approximately 80% of human GBMs and correlates with the fact that RTKs/PI3K/Akt signaling is altered in 88% of GBM (TCGA Research Network, Nature 455(23): 1061-68). Oncogenic Akt mutations have not been detected in GBM. Akt inhibitor perifosine is undergoing clinical evaluation in malignant gliomas (Nakada, M. et al., Cancers, 2011, 3: 3242-3278).
2. p14ARF/MDM2/p53 Pathway
The p53 gene encodes a protein that responds to diverse cellular stresses to regulate target genes that induce cell cycle arrest, cell death, cell differentiation, senescence, DNA repair, and neovascularization. Following DNA damage, p53 is activated and induces transcription of genes (such as p21Waf1/Cip1) that function as regulators of cell cycle progression at G1 phase. Mouse double minute 2 homolog (MDM2) oncogne inhibits p53 transcriptional activity by forming a tight complex with the p53 gene, and participates in the degradation of p53. The p14ARF gene codes a protein that directly binds to MDM2 and inhibits MDM2-mediated p53 degradation. In turn p14ARF expression is negatively regulated by p53. Thus, inactivation of p14ARF/MDM2/p53 is caused by altered expression of any of the p53, MDM2, or p14ARF genes. The p53 pathway plays a crucial role in the development of secondary GBMs. The p53 gene is the most commonly mutated p53 pathway gene in glioma; however, molecular abnormalities involving other genes in the pathway have also been described. (Nakada, M. et al., Cancers, 2011, 3: 3242-3278). The p14ARF/MDM2/p53 Pathway is shown in FIG. 2.
3. RB Pathway.
The RB (retinoblastoma tumor suppressor protein) pathway suppresses cell cycle entry and progression, as well as the p53 pathway. The 107-kDa RB1 protein encoded by RB1 (at 13q14) controls progression through G1 into the S-phase of the cell cycle (Serrano, M., et al., Nature, 1993, 366: 704-707). The CDKN2A protein (i.e. p16INK4a which is cyclin-dependent kinase inhibitor 2A) binds to cyclin-dependent kinases 4 (CDK4) and inhibits the CDK4/cyclin D1 complex, thus inhibiting cell cycle transition from G1 to S phase. Thus, alteration of RB1, CDK4, or CDKN2A can cause dysregulation of the G1-S phase transition. However, alteration of only the RB pathway is insufficient to induce tumor formation. EGFR amplification enhances the PI3K pro-growth pathway and is typically associated with CDKN2A deletions. CDKN2A loss is associated with the classical subtype of GBM, according to the TCGA study. (Nakada, M. et al., Cancers, 2011, 3: 3242-3278). The RB pathway is shown in FIG. 3.
4. Ras/MEK/MAPK Pathway.
RAS (Rat sarcoma) proteins act as on/off (RAS-GDP/RAS-GTP) switches controlled by RTKs and neurofibromatosis type 1 tumor suppressor gene (NF-1). Activated RAS (RAS-GTP) then activates serine/threonine kinase RAF. RAF activates mitogen-activated protein kinase kinase (MAPKK), also called MEK, which in turn activates MAPK. MAPK activation results in activation of various transcription factors, such as Elk1, c-myc, Ets, STAT1/3, and PPAR.
The NF-1 tumor suppressor gene encodes neurofibromin, which functions primarily as a RAS negative regulator and plays a role in adenylate cyclase- and Akt-mTOR-mediated pathways. There is increasing evidence that the NF-1 gene is involved in the tumorigenesis of not only NF-1-related, but also sporadically occurring, gliomas. In the TCGA pilot study, NF-1 mutation/homozygous deletions were identified in 18% of GBM. Mesenchymal GBMs, having frequent inactivation of the NF-1 (37%), p53 (32%), and PTEN genes, respond to aggressive chemo-radiation adjuvant therapies. (Nakada, M. et al., Cancers, 2011, 3: 3242-3278). The Ras/MAPK pathway is shown in FIG. 4. A global view of the signaling pathways mentioned above is shown in FIG. 4.
Other signaling pathways may play a role in GBM initiation, migration, and invasion.
Brain Stem Cells
Glial cells outnumber neurons by 10-fold in the human brain and are composed mainly of terminally differentiated cells and minor discrete precursor populations. (Ikanizadeh, S. et al., “Glial Progenitors as targets for transformation in glioma,” Adv. Cancer Res. 121: 1-65 (2014)).
Two major germinal layers—the ventricular zone (VZ) and the subventricular zone (SVZ)— give rise to most neurons and glial cells in the forebrain. (Garcia-Verdugo, J M et al, “Architecture and Cell types of the adlt subventricular zone: in search of the stem cells,” J. Neurobiol. 36: 234-48 (1998)). It was traditionally believed that the capacity of these germinal layers to generate neurons was restricted to the embryonic period; however, it is now known that new neurons continue to be added to certain regions of the adult vertebrate brain. In adult mammals, neuronal addition has been observed only in the olfactory bulb and the hippocampus. New neurons destined for the olfactory bulb are born in the SVZ of the lateral ventricles. A subpopulation of SVZ cells can proliferate in culture, giving rise to spherical clusters of cells (neurospheres), which have the capacity to generate neurons, astrocytes and oligodendrocytes. Based on their ability to self-renew and their potential to give rise to multiple cell types, these SVZ-derived cells are considered neural stem cells. Evidence also suggests that neural stem cells (NSCs) line the third and fourth ventricles (Ikanizadeh, S. et al., “Glial Progenitors as targets for transformation in glioma,” Adv. Cancer Res. 121: 1-65 (2014), citing Weiss, S. et al, “Multipotent CNS stem cells are present in the adult mammalian spinal cord and ventricular neuroaxis,” J. Neurosci. 16(23): 7599-7609 (1996); Xu, Y. et al., Neurogenesis in the ependymal layer of the adultrat 3rd ventricle,” Exptl Neurol. 192(2): 251-64 (2005)).
Modeling of glioma in mice has shown that cells at various differentiation stages throughout glial and neuronal lineages have the potential to generate glioma. Recent advances highlight the cellular heterogeneity in gliomas, the influence of the tumor microenvironment, and that treatment-resistant tumor cells display a high degree of stemness.
Transcriptomal profling of gliomas displaying a neuroepithelial origin, show that the mesenchymal phenotype is associated with stemness, invasiveness, and poor survival. Id. (citing H. S. Phillips, et al., “Molecular subclasses of high-grade glioma predict prognosis, delineate a pattern of disease progression, and resemble stages in neurogenesis,” Cancer Cell. 9(3): 157-73 (2006); Sturm, D. et al, “Hotspot mutations in H3F3A and IDH1 define distinct epigenetic and biological subgroups of glioblastoma,” Cancer Cell 22(4): 425-37 (2012); Verhaak, R G W, et al., “Integrated genomic analysis identifies clinically relevant subtypes of glioblastoma characterized by abnormalities in PDGFRA, IDH1, EGFR, and NF1,” Cancer Cell. 17(1): 98-110 (2010)).
Cancer stem cells (CSCs) or tumor-initiating cells (TIC) are a subpopulation of tumor cells with the ability to undergo self-renewal and recapitulate the entire tumor population Wang, L. et al., Interleukin-1β and transforming growth factor-β cooperate to induce neurosphere formation and increase tumorigenicity of adherent LN-229 glioma cells,” Stem ell Res. & Therapy 3:5 (2012). Glioma stem cells (GSC) have been identified from human glioma tissues and glioma cells lines. Id. GSCs are characterized by the ability of self-renewal to generate three-dimensional aggregates of cells in suspension termed “neurospheres” when cultured in serum-free conditions supplemented with epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). Id. These glioma neurospheres reflect biological and pathological characteristics of primary gliomas, display resistance to chemo- and radio-therapies, and have enhanced oncogenic potential, generating tumors that reproduce the characteristics of the original tumors after intracranial transplantation. (Id., citing Ehrlicher, A. et al., Guiding neuronal growth with light,” Proc. Natl Acad. Sci. U.S.A. 99: 16024-8 (2002); Difato, F. et al, “combined optical tweezers and laser dissector for controlled ablation of functional connections in neural networks,” J. Biomed. Opt. 16: 051306 (2011); Dictus, C. et al, “Comparative analysis of in vitro conditions for rat adult neural progenitor cells,” J. Neurosci Methods. 161: 250-58 (2007))
Animal Models of Glioma
The study of IDH-mutant gliomas has been obstructed by the lack of models of IDH-mutant glioma-producing mice. Brain-specific IDH1R132H knock-in mice are embryonically lethal. Izanizadeh, S. et al, citing Sasaki, M. et al, “D-2 hydroxyglutarate produced by mutant IDH1 perturbs collagen maturation and basement membrane function,” Genes & Devel. 26(18): 2038-49 (2012)). Cell lines with IDH1R132H mutation can only be maintained transiently in vitro, since the mutation does not persist in non-immortalized cells. Primary IDH-mutant gliomas from patient tumors do not grow well in vitro (Id., citing Piaskowski, S. et al., “Glioma acells showing IDH1 mutation cannot be propagated in standard cell culture conditions,” Br. J. Cancer 104(6): 968-70 (2011)). In contrast to normal cells, introduction of IDH mutations into glioma cells decreases the proliferation rate, which may ultimately cause a selection pressure against cultured glioma cells harboring IDH mutations (Id., citing Bralten, L B C, et al., “IDH1 R132H decreases proliferation of glioma cell lines in vitro and in vivo,” Annals Neurol. 69(3): 455-63 (2011)).
Spontaneous mouse models of GBM have been generated that are caused by mutation and therefore loss of three glioma relevant tumor suppressor genes: Pten, p53 and NFL These mice have tumors that exhibit histopathological and molecular similarity with human GBM and have provided a powerful platform for natural history studies, molecular studies and derivation of primary (Mut6) cells that can be maintained in low passage culture and reintroduced in allografts to produce GBM (Llaguno S A et al., “Malignant Astrocytomas Originate from Neural Stem/Progenitor Cells in a Somatic Tumor Suppressor Mouse Model”, Cancer Cell. 2009 Jan. 6; 15(1): 45-56; Llaguno S A et al., “Neural and Cancer Stem Cells in Tumor Suppressor Mouse Models of Malignant Astrocytoma”, Cold Spring Harb Symp Quant Biol. 2008; 73: 421-426).